1. Field of the Invention
The present invention generally relates an improved system and method for stimulating multiple production zones in a wellbore. U.S. Pat. No. 6,006,838 discloses a string that includes modules with sliding sleeves that may be used to stimulate multiple production zones in a wellbore in a single trip into the wellbore. The present invention discloses positioning the string disclosed in U.S. Pat. No. 6,006,838 within a desired location within a wellbore and then cementing the string in place using an acid soluble cement. Cement is pumped down the string, out the end of the string, and up and around the outside of the diameter of the string. The cement is allowed to cure cementing the string at the desired location.
A wiper plug is pumped down the string after the cement, and preferably before the displacement fluid, to wipe any residual cement from the inner diameter of the string. The wiper plug also helps to separate the acid soluble cement from acid pumped down the string after the wiper plug. At least one wiper ball may also be pumped down the string after the wiper plug. The wiper ball may be pumped down the string within a spacer fluid to help protect the wiper ball from being damaged by the acid solution. The wiper ball may help to remove any residual cement from the internal bores of the modules allowing the sliding sleeves to slide when actuated. The acid pumped within the string also prevents any residual cement from curing inside of the string.
After the cement has cured around the outside of the string, fluid is pumped down the string. The hydraulic pressure of the pumped fluid moves the sliding sleeve of the lowermost module to an open position. The acid within the string breaks down the cement around the string after the sliding sleeve of a module is opened. Hydraulic pressure may then fracture the formation adjacent the opened module. A proppant containing slurry may follow behind the acid to extend and support the fracture. Once the formation has been fractured an appropriately sized ball may be dropped down the string to land in the ball seat of the next lowermost module. The seated ball prevents flow to the first module and the pressure within the string will build until the sliding sleeve of the second module moves to the open position. The acid then breaks down the cement adjacent to the second module and hydraulic pressure may fracture the formation at this location. The process is repeated until cement adjacent each module has been broken down and each of the specified zones have been fractured.
2. Description of the Related Art
This present invention relates to an improved system and method for stimulating producing zones of an openhole wellbore in oil and gas wells. Previously disclosed was an assembly for selectively stimulating a wellbore without the use of openhole inflatable packers. This assembly is especially suited to perform a combination of matrix acidizing jobs and near wellbore erosion jobs at a number of producing zones in the wellbore in a single trip.
Prior to the disclosed assembly, operators who were interested in stimulating multiple producing zones in an openhole wellbore could stimulate the zones one zone at a time by using a workstring and an openhole inflatable packer. Such a method and assembly required the operator to set an inflatable packer (or other similar apparatus) above each zone of interest to be stimulated and then, following the stimulation job, to release the packer (or packers) and trip the packer assembly to a new location where it would be reset for the next stimulation job. This procedure would be repeated for each desired zone of interest. However, because of the tripping time and the difficulty in setting and maintaining the seal in inflatable packers in openhole wellbores, such a method was both time consuming and relatively unreliable. Furthermore, openhole inflatable packers (or other similar devices) are expensive to rent or to purchase. As a result of the relative unreliability and cost of using openhole inflatable packers, such assemblies prove to be uneconomical in marginal fields such as fields in the Permian Basin region of West Texas and Eastern New Mexico.
The previously disclosed assembly does not require an inflatable packer and is very economical to build and maintain. Thus, an operator can use the assembly for a small incremental cost over what it costs to perform an acid job and receives the benefits of not only a matrix acidizing treatment, but can also enhance the flow in the near wellbore region by eroding away near wellbore skin damage. The assembly also allows an operator to accurately position an assembly in a wellbore to ensure that the producing zones of interest are stimulated.
The present invention is an improvement to the previously disclosed assembly for selectively stimulating a wellbore without the use of openhole inflatable packers. Specifically, the system and method is disclosed for cementing the assembly at a desired location within the wellbore. The use of an acid soluble cement pumped down the string of the assembly allows the assembly to be cemented in place within a wellbore. The use of a wipe plug and at least one wiper ball removes any residual cement from the inside of the string. The acid soluble cement also provides that the use of an acid within the string prevents the curing of any residual cement within the string.